Field of the Invention
This invention relates to a hybrid vehicle, and particularly to a hybrid vehicle including first and second rotating electric machines and an internal combustion engine.
Description of the Background Art
A hybrid vehicle is known that is configured to include, in addition to an engine, two rotating electric machines and a power split device, a transmission mechanism between the engine and the power split device.
The vehicle disclosed in WO2013/114594 adopts a series/parallel hybrid system. In the vehicle using the series/parallel hybrid system, motive power of the engine is transmitted to a first motor generator (first MG) to be used for generating electric power, while part of the motive power of the engine is transmitted to a driving wheel via the power split device.
A hybrid vehicle is also known that is configured to perform series running by generating electric power using the motive power of the engine, and driving the motors using the generated electric power (series hybrid system). With this series hybrid system, the motive power of the engine is not transmitted to the driving wheel.
The vehicle disclosed in the above-mentioned document has a configuration such that it cannot perform series running because, when the motive power of the engine is transmitted to the first motor generator (first MG), it is also transmitted to the driving wheel via the power split device.
With the series/parallel hybrid system, when the vehicle speed is low, for example, torque variations in the engine may cause rattling noise to be generated by a gear mechanism disposed in a drive device between the engine and the driving wheel. It is thus necessary to select an operating point of the engine to prevent generation of such rattling noise, and the engine is sometimes operated at an operating point not optimal in terms of fuel efficiency. Thus, there has been room for improvement in terms of fuel efficiency.
On the other hand, with the series system, the engine and the gear mechanism disposed in the drive device are completely decoupled from each other, without the need to take such rattling noise into much consideration. However, all the torque of the engine is converted to electric power once, and then converted back to a torque for the driving wheel by the motors. This makes the series system inferior to the series/parallel hybrid system in terms of fuel efficiency, within a range of speeds where the engine has good operating efficiency.
As described above, because the series/parallel hybrid system is superior to the series hybrid system in some ways, it would be desirable to have a configuration such that series running or series/parallel running can be selected depending on the conditions of the vehicle.
In order to realize such a hybrid vehicle capable of both series running and series/parallel running, a configuration may be provided that includes a clutch directly connecting the engine and the first motor generator. In a series running state where the engine and the first motor generator are directly connected, electric power resulting from regenerative braking generated by a second motor generator during braking is consumed by the causing first motor generator to continue cranking the engine that is not self-rotating. This state can also be regarded as one mode where an engine brake is being used. That is, because the engine is caused to idle, a friction brake need not be used for running on a long downhill, for example.
In particular, the engine brake may be used when charging of a power storage device is to be restricted, which restricts the use of a regenerative brake. If, however, the clutch connecting the engine and the first motor generator is engaged after reception of a deceleration request from a driver, the start of the engine brake would be delayed. In particular, at low temperatures where the clutch response becomes poor, the operation of the engine brake need to be started earlier.